1. Field of the Invention
The present invention relates in general to the process of fabricating semiconductor structures by microphotolithography and, more particularly, by multiple exposures for pitch multiplication.
2. Description of the Prior Art
The advances of microelectronic manufacture are reflected by the density and dimensions of semiconductor structures created by the microphotolithographic process. The demand for high density and small critical dimension (CD) has been constantly pushing photolithography technology to its limits. At the limits of a photolithographic process, features with relaxed pitch can be created with a smaller CD than that with high pitches, e.g., 1:1 line/space. The trade-off of such processes for smaller CDs is the reduction of feature density. By principle, the loss of density can be compensated for by repeating the exposure process.
The potential for smaller CDs had not been seriously investigated until recently, primarily for two reasons: (1) the high cost and high complexity of the multiple exposure process, and (2) the availability of other options for CD reduction. In the last 10 years, the microelectronics industry primarily relied on shorter radiation wavelengths of exposure tools for smaller CDs. The photolithography technology has successfully evolved from i-line (365 nm) to KrF (248 nm) and ArF (193 nm).
Patterns of 45-nm line/space with 1:1 pitch can be printed reliably with 193-nm immersion photolithography. However, as immersion photolithography quickly reaches its resolution limit, trends toward improving the photolithography process have included the use of high numerical aperture (NA) tools and/or immersion fluids. Using imaging tools with high NA capabilities (>1.0) by themselves or in combination with immersion provides a method to achieve higher resolution of patterns with smaller critical dimension and higher density. These advances are possible because of the larger amount of light that can be transferred to the imaging layer. However, these options are quite costly and require new tool sets.
More recently, multiple exposure technology for the next printing node has become the only viable option until exposure wavelengths shorter than 193 nm, such as 13.5 nm, are available. Many process schemes for multiple exposure technology have been investigated and reported. Most of these schemes utilize a bright field mask. In another words, only small portions of the photoresist, such as lines, are protected from the exposure, while the remaining portion of the resist is exposed. The photoresist is then contacted with developer to remove the exposed portions of the resist, thereby leaving only the unexposed portion of the photoresist (i.e., the lines) remaining above the hardmask layer. The pattern is transferred to the hardmask by etching away the hardmask layer except for those areas underneath the unexposed portions of the photoresist. The process is repeated until the desired pattern is achieved. One drawback to the traditional bright field process is that a hardmask must be reapplied to the substrate before the second exposure-development-etching process. This additional step increases processing time as well as overall cost. Little attention has been paid to processes utilizing a dark field mask to form features such as vias or trenches. In a dark field exposure process, a large portion of the photoresist is protected from exposure, while only the small portions of the photoresist are exposed and removed after development. As with bright field, the pattern must then be transferred to the hardmask using an etching process.
Thus, existing double exposure processes require a dry-etch step between the two exposures. In other words, the patterns achieved from the first exposure must be transferred to the underlying layer by reactive ion etching (RIE) before the second exposure can be processed. The dry-etch step greatly complicates the double exposure technology. Accordingly, there is a need in the art for multiple patterning techniques that do not require the application of a second hardmask layer, and also eliminate the dry-etch step.